Ceramics honeycomb structural body and method of manufacturing the structural body

ABSTRACT

The ceramics honeycomb structure of the present invention is formed of a plurality of cells forming a fluid flow passage partitioned by porous partition walls, and comprising an inflow end part allowing fluid to flow therein, an outflow end part allowing fluid to flow therefrom, and an outer peripheral part including an outer peripheral surface, and is characterized by having a structure where a porosity per unit volume (cm 3 ) gradually increases from the inflow end part side to the outflow end part side at a rate of 0.2%/mm or less, and this ceramics honeycomb structure has an excellent erosion resistance of partition walls positioned at a cell opening end part and a high compressive strength (isostatic strength) at the time of canning, and is suitable particularly as an automobile exhaust gas purification catalyst carrier.

TECHNICAL FIELD

The present invention relates to a ceramics honeycomb structure having aporous structure and a method of manufacturing the same. Moreparticularly, it relates to a ceramics honeycomb structure which has anexcellent erosion resistance at its honeycomb end face and is wellbalanced in characteristics such as purification performance and canningproperties and has the characteristics suitable, particularly, ascarriers for automobile exhaust gas purification catalysts, and furtherrelates to a method of manufacturing the ceramics honeycomb structure.

BACKGROUND ART

Ceramics honeycomb structures widely used for exhaust gas purificationcatalyst carriers are required to have a higher purification performancefor coping with regulations on exhaust gas which have been yearlytightened and are further required to be reduced in pressure loss forreduction of fuel cost and increase of output.

Under the circumstances, there are increasing trends to increase theopening ratio at a cell opening end face of the honeycomb structures toreduce the pressure loss by thinning the thickness of partition walls ofthe honeycomb structures and to improve purification performance byreducing heat capacity of partition walls to cause early activation ofcatalyst after starting of engine.

On the other hand, with thinning of the partition walls of honeycombstructures, there is a new problem of occurrence of such an erosionphenomenon that various foreign matters incorporated in exhaust gasstrike against the partition walls positioned at a cell opening end partto damage the partition walls.

For this problem, there has already been proposed a honeycomb structurein which the partition walls positioned at the cell opening end parthave a partition wall reinforced portion (reinforced partition wallportion) increased in strength than other partition wall portions (forexample, see patent document 1), and furthermore various investigationshave been made on the method of providing the reinforced partitionportion.

Hitherto, as a method of providing the partition wall reinforcedportion, it has been known that a substrate having a honeycomb structuremainly composed of a raw material convertible into cordierite is fired,then a slurry prepared by dispersing the a raw material convertible intocordierite in a dispersion medium is applied to partition wallspositioned at a cell opening end part of the substrate, and thereafterthe substrate is dried and fired (for example, see patent document 1).

However, this method requires firing step which needs a long time at twostages, namely, firing of the substrate and firing for providing thepartition wall reinforced portion, and thus has serious problems inproduction efficiency and production cost.

On the other hand, a method has been suggested which comprises applyinga slurry prepared by dispersing a partition wall reinforcing material ina dispersion medium to partition walls positioned at the cell openingend part at a stage before firing the substrate of honeycomb structure,and then drying and firing the substrate, whereby firing of thesubstrate and formation of the partition wall reinforced portion areperformed by one firing (for example, see patent document 1).

However, as to this method, no specific investigation is conducted onthe difference in composition before and after the firing of thesubstrate. Especially, the substrate before firing usually containsorganic binders and the like added for the purpose of improving strengthof the partition walls, but no consideration has been given to the factthat most of the organic binders are water-soluble compounds such asmethyl cellulose.

Therefore, if the step of formation of the partition wall reinforcedportion which has conventionally been carried out after firing iscarried out before firing using a slurry prepared by dispersing thepartition wall reinforcing material in water, the organic binderdissolves in the slurry to cause distortion of partition walls of theresulting honeycomb structure to cause reduction of isostatic strength,and thus the honeycomb structure cannot be practically used.

Furthermore, in the case of using a slurry prepared by dispersing apartition wall reinforcing material in a dispersion medium, thepartition wall reinforcing material settles or agglomerates due to itsphysical properties, and dispersibility of the partition wallreinforcing material is apt to be insufficient. Thus, variation orununiformity of reinforcing degree tends to occur in the partition wallreinforced portion formed. Therefore, this production method suffersfrom the problems that a ceramic honeycomb structure having a uniformerosion resistance in the whole partition wall reinforced portion cannotbe stably obtained, and besides burden of control for attaining uniformdispersion of the partition wall reinforcing material increases.

On the other hand, if a slurry prepared by dispersing a partition wallreinforcing material in a water-insoluble dispersion medium is used, theproblem of reduction in isostatic strength caused by distortion ofpartition walls and others can be solved.

However, even this production method cannot solve the problems that aceramic honeycomb structure having a uniform erosion resistance in thewhole partition wall reinforced portion cannot be stably obtained, andbesides burden of control for attaining uniform dispersion of thepartition wall reinforcing material in the slurry increases.

Furthermore, with progress in thinning of the wall thickness ofhoneycomb structures as mentioned above, compressive strength (isostaticstrength) of honeycomb structures at the time of canning considerablydecreases and the honeycomb structures sometimes cannot be practicallyused.

(Patent document 1)

JP-A-2000-51710

The present invention has been made in view of the above problems in theconventional technologies, and the object is to provide a ceramicshoneycomb structure which has an excellent erosion resistance inpartition walls positioned at a cell opening end part and a highcompressive strength (isostatic strength) at the time of canning and issuitable particularly as an automobile exhaust gas purification catalystcarrier, and a method for producing the ceramics honeycomb structure.

DISCLOSURE OF INVENTION

That is, according to the present invention, there is provided aceramics honeycomb structure which is formed of a plurality of cellsforming a fluid flow passage partitioned by porous partition walls andhas an inflow end part allowing fluid to flow therein, an outflow endpart allowing fluid to flow therefrom, and an outer peripheral partincluding an outer peripheral surface as its respective parts,characterized by having such a structure that a porosity per unit volume(cm³) gradually increases from the inflow end part side to the outflowend part side at a rate of 0.2%/mm or less (the first invention).

In the present invention, it is preferred that the ceramics honeycombstructure has such a structure that the porosity per unit volume (cm³)gradually increases from the inflow end part side to the outflow endpart side at a rate of 0.1%/mm or less.

Furthermore, according to the present invention, there is provided aceramics honeycomb structure which is formed of a plurality of cellsforming a fluid flow passage partitioned by porous partition walls andhas an inflow end part allowing fluid to flow therein, an outflow endpart allowing fluid to flow therefrom, and an outer peripheral partincluding an outer peripheral surface as its respective parts,characterized by having such a structure that a porosity per unit volume(cm³) gradually decreases from the central part of a sectionperpendicular to the flow passage of the cells to the outer peripheralpart at a rate of 0.2%/mm or less (the second invention).

In the present invention, it is preferred that the ceramics honeycombstructure has such a structure that the porosity per unit volume (cm³)gradually decreases from the central part of a section perpendicular tothe flow passage of the cells to the outer peripheral part at a rate of0.1%/mm or less.

In the present invention, it is preferred that a porosity per unitvolume (cm³) within the area of up to 150 mm from the flow passage endface of the inflow end part side in the inward direction of the flowpassage is 10-50%.

In the present invention, it is preferred that the minimum thickness ofthe partition walls is 0.030-0.076 mm, and the ceramics honeycombstructure comprises at least one ceramics selected from the groupconsisting of cordierite, alumina, mullite, silicon nitride, aluminumtitanate, zirconia and silicon carbide.

In the present invention, it is preferred that the section perpendicularto the flow passage has a shape of circle, ellipse, oval, trapezoid,triangle, tetragon, hexagon or left-right asymmetrical irregular shape,and the section of the cells perpendicular to the flow passage has ashape of triangle, tetragon or hexagon.

The ceramics honeycomb structure of the present invention is preferablyused for automobile exhaust gas purification catalyst carriers, andpreferably the ceramics honeycomb structure in which a catalystcomponent is supported on the partition walls is incorporated into acatalyst converter by being held at the outer peripheral surface of theouter wall.

Further, according to the present invention, there is provided a methodfor producing a ceramics honeycomb structure, characterized in that adried substrate having a honeycomb structure is obtained using a claymainly composed of a ceramics material, the substrate is coated with andimpregnated with a reinforcing agent mainly composed of a compoundhaving in its structure at least one element selected from the groupconsisting of Si, Ti, Mg and Al, and thereafter the substrate is fired(the third invention).

Moreover, according to the present invention, there is provided a methodfor producing a ceramics honeycomb structure, characterized in that adried substrate having a honeycomb structure is obtained using a claymainly composed of a ceramics material, the substrate is fired to obtaina fired substrate, the resulting fired substrate is coated with andimpregnated with a reinforcing agent mainly composed of a compoundhaving in its structure at least one element selected from the groupconsisting of Si, Ti, Mg and Al, and thereafter the substrate is firedagain (the fourth invention).

In the present invention, said compound is preferably one which producesan inorganic oxide upon burning, and more preferably one which has asiloxane bond. In the present invention, said compound is preferably asilicone oil, a silicone varnish, an alkoxy oligomer or a mixturethereof. In the present invention, the reinforcing agent preferably hasan absolute viscosity of 1-10000 mPa·s.

In the present invention, the ceramics material is preferably a rawmaterial convertible into cordierite, and the clay preferably contains awater-soluble organic binder. This water-soluble organic binderpreferably comprises at least one water-soluble compound selected fromthe group consisting of hydroxypropylmethyl cellulose, methyl cellulose,hydroxyethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol andpolyvinyl acetal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side view of a ceramics honeycomb structure whichshows the parts from which samples are taken (A-E) (hereinafter referredto as sampling parts).

FIG. 2 is a schematic view of a section perpendicular to the flowpassage of cells of a ceramics honeycomb structure which shows thesampling parts (F-J).

FIG. 3 is a graph in which porosity (%) is plotted against the samplingparts (mm) on the ceramics honeycomb structures produced in examples andcomparative examples.

FIG. 4(a)-FIG. 4(c) schematically show one embodiment of the ceramicshoneycomb structure of the present invention (the first invention), andFIG. 4(a) is an oblique view, FIG. 4(b) is a plan view and FIG. 4(c) isa side view.

FIG. 5 is a partially enlarged view which shows another embodiment ofthe ceramics honeycomb structure of the present invention (the first andsecond inventions).

FIG. 6 schematically shows an example in which the ceramics honeycombstructure is incorporated in a converter container.

FIG. 7 is a graph which shows the condition of engine speed in erosiontest.

FIG. 8 schematically shows the method for measurement of erosion amount.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the present invention will be explained below. Theyshould not be construed as limiting the invention in any manner, and itshould be understood that optional changes, modifications andimprovements may be made based on the normal knowledge of one skilled inthe art without departing from the spirit and scope of the invention.

One embodiment of the ceramics honeycomb structure of the presentinvention (the first invention) is a ceramics honeycomb structure formedof a plurality of cells forming a fluid flow passage partitioned byporous partition walls and having an inflow end part allowing fluid toflow therein, an outflow end part allowing fluid to flow therefrom, andan outer peripheral part including an outer peripheral surface,characterized by having such a structure that a porosity per unit volume(cm³) gradually increases from the inflow end part side to the outflowend part side at a rate of 0.2%/mm or less. A detailed explanation willbe given below.

FIG. 4(a)-FIG. 4(c) schematically show one embodiment of the ceramicshoneycomb structure of the present invention (the first invention), andFIG. 4(a) is an oblique view, FIG. 4(b) is a plan view and FIG. 4(c) isa side view. The ceramics honeycomb structure 1 is formed of a pluralityof cells 3 partitioned by porous cell partition walls 2, and has a cellopening end part 5 (an inflow end part) allowing fluid to flow therein,a cell opening end part 5 (an outflow end part) allowing fluid to flowtherefrom, and an outer wall 4 as its respective parts. In the ceramicshoneycomb structure 1 of this embodiment, the porosity per unit volume(cm³) of the inflow end part side is lower than that of the outflow endpart side between the cell opening end parts 5, namely, the inflow endpart side has denser microstructure. Therefore, the ceramics honeycombstructure of the present invention has the advantages that problems suchas increase of heat capacity and deterioration of catalystsupportability with increase of weight hardly occurs as compared with aceramics honeycomb structure which is reduced in porosity uniformlythrough the structure. The reference numeral 10 in FIG. 4(c) indicatesthe opening end part.

Furthermore, a case is supposed where the ceramics honeycomb structureof this embodiment is used as a carrier for exhaust gas purificationcatalyst by disposing the inflow end part lower in porosity per unitvolume (cm³) at the inflow side (upstream side) of the exhaust gas whichis a fluid to be treated. In this case, when foreign matters (such asoxidation scales) present in the exhaust gas strike against thepartition walls, the walls are hardly damaged because the porosity perunit volume (cm³) of the partition walls is lower than that of thepartition walls disposed on the downstream side, and thus occurrence oferosion phenomenon can be inhibited.

From the viewpoint of more effective inhibition of the erosionphenomenon, it is preferred for the ceramics honeycomb structure of thisembodiment that the porosity per unit volume (cm³) gradually increasesfrom the inflow end part side to the outflow end part side at a rate of0.1%/mm or less.

Next, an embodiment of the second invention will be explained. Oneembodiment of the second invention is a ceramics honeycomb structurewhich is formed of a plurality of cells forming a fluid flow passagepartitioned by porous partition walls and has as its respective parts aninflow end part allowing fluid to flow therein, an outflow end partallowing fluid to flow therefrom, and an outer peripheral part includingan outer peripheral surface, characterized by having such a structurethat a porosity per unit volume (cm³) gradually decreases from thecentral part of a section perpendicular to the flow passage of the cellstowards the outer peripheral part at a rate of 0.2%/mm or less. Thedetails will be explained below.

In the ceramics honeycomb structure of this embodiment, the porosity perunit volume (cm³) of the outer peripheral part is lower than that of thecentral part, namely, the outer peripheral part has a densermicrostructure. Therefore, the ceramics honeycomb structure of thepresent invention has the advantages that problems such as increase ofheat capacity and deterioration of catalyst supportability with increaseof weight hardly occur as compared with a ceramics honeycomb structurewhich is decreased in porosity uniformly through the structure.

A case is supposed where the ceramics honeycomb structure of thisembodiment is used by holding (canning) it in a suitable holdingcontainer, holding jig, etc. In this case, since the porosity per unitvolume (cm³) of the outer peripheral part which contacts with theholding container, jig, etc. is lower than that of the central part, theceramics honeycomb structure has a sufficient isostatic strength andhence can be surely held in the holding container, holding jig, etc. andproblems such as damaging caused by load of compressive face pressurehardly occur. The term “isostatic strength” here means a strength shownby a pressure value applied at breaking in the test in accordance withautomobile standard JASO M505-87.

From the viewpoint of more effectively avoiding the problems such asincrease of heat capacity due to increase of mass and decrease ofcatalyst supporting properties, it is preferred for the ceramicshoneycomb structure of this embodiment that the porosity per unit volume(cm³) gradually decreases from the central part of a sectionperpendicular to the flow passage of the cells to the outer peripheralpart at a rate of 0.1%/mm or less.

Assuming that the ceramics honeycomb structures of the first and secondembodiments are used mainly as carriers for catalysts for purificationof exhaust gas, from the viewpoints of catalyst supportability andisostatic strength, the porosity per unit volume (cm³) in the area of upto 150 mm from the flow passage end face of the inflow end part side inthe inward direction of the flow passage is preferably 10-50%, morepreferably 15-45%, especially preferably 20-40%.

The thickness of the partition walls of the ceramics honeycombstructures of the first and second embodiments is not particularlylimited, but it is preferably thinner for the improvement ofpurification performance at the time of warming up by weight-saving andreduction of heat capacity while reducing pressure loss, andspecifically it is preferably 0.030-0.076 mm, more preferably0.030-0.065 mm in minimum partition wall thickness. Furthermore, in thecase of the ceramics honeycomb structures of the first and secondembodiments, even when the minimum partition wall thickness is less than0.050 mm, there is substantially no possibility of occurrence ofdeformation in partition walls.

In the first and second embodiments, it is also preferred for improvingerosion resistance to increase the thickness of cell partition walls 2 aon the outer peripheral part side as shown in FIG. 5. Moreover, byincreasing the thickness of cell partition walls 2 a on the outerperipheral part side, isostatic strength can be improved and holdingforce at the time of canning can also be increased to improve canningproperties. The isostatic strength here is a strength expressed by apressure value applied at breaking in the test in accordance withautomobile standard JASO M505-87. In FIG. 5, outermost peripheral cells8 are present closest to the outer wall 4, and the second cells 9 arepresent inwardly continuing from the outermost peripheral cells 8. Thethickness of the partition walls of the outermost peripheral cells isshown by Tr₁ and the thickness of the partition walls of the cells 9positioned inwardly second from the outermost peripheral cells 8 isshown by Tr₂. Although not shown, thickness of partition walls of the5th to 15th cells are similarly shown by Tr₅₋₁₅. The cell partitionwalls 2 are divided broadly into outer peripheral cell partition walls 2a and basic cell partition walls 2 b.

In the embodiments of the first and second inventions, when theoutermost peripheral cells are called starting point cells, the cells inthe range of the 3rd to 20th cells positioned on the inside of thestarting point cells are called end point cells, and the cellspositioned on the inside of the end point cells are called basic cells,it is preferred that the thickness (Tr₁, Tr₃₋₂₀) of the partition wallsforming the starting point cells and the end point cells and thethickness (Tc) of the partition walls forming the basic cells satisfythe relation: 1.10≦(Tr₁, Tr₃₋₂₀)/Tc≦3.00. If this value [(Tr₁,Tr₃₋₂₀)/Tc] is less than 1.10, it does not contribute to the improvementof erosion resistance and the improvement of isostatic strength, andthus it does not contribute to the improvement of canning properties. Ifthe value exceeds 3.00, heat capacity and pressure loss increase. Evenif the thickness (Tr₁, Tr₂) of partition walls of the first and secondcells is increased at a specific rate, it does not contributes to theimprovement of erosion resistance or isostatic strength, and if thethickness of partition walls of the 21st and the following cells,particularly, the 31st and the following cells, is increased at aspecific rate, the pressure loss increases and furthermore the heatcapacity also increases due to the increase of mass of the ceramicshoneycomb structure to more than a specific value.

Moreover, in the embodiments of the first and second inventions,considering the heat capacity and pressure loss, it is practicallypreferred to further limit the conditions in such a manner that thethickness of the partition walls (Tr₁, Tr₃₋₂₀) and the thickness of thebasic cells (Tc) have the relation: 1.10≦(Tr₁, Tr₃₋₂₀)/Tc≦2.50,furthermore, 1.20≦(Tr₁, Tr₃₋₂₀)/Tc≦1.60.

The ceramics honeycomb structures of the embodiments of the first andsecond inventions comprise, for example, at least one ceramics selectedfrom the group consisting of cordierite, alumina, mullite, siliconnitride, aluminum titanate, zirconia and silicon carbide.

Furthermore, it is preferred that the section perpendicular to the flowpassage of the ceramics honeycomb structures of the embodiments of thefirst and second inventions has a shape of, for example, circle,ellipse, oval, trapezoid, triangle, tetragon, hexagon or irregular shapeasymmetrical in left and right. Among them, circle, ellipse or oval ispreferred.

The section of the cells perpendicular to the flow passage of theceramics honeycomb structures of the embodiments of the first and secondinventions has a shape of polygon such as triangle, square, rectangle,and hexagon, and preferred is triangle, tetragon or hexagon.

The ceramics honeycomb structures of the embodiments of the first andsecond inventions are not particularly limited in their uses, and can beused for various uses such as various filters and catalyst carriers, andespecially preferably they are used as carriers for automobileexhaustion gas purification catalysts. Moreover, the ceramics honeycombstructures of the embodiments of the first and second inventions arepreferably used by incorporating in a catalyst converter container 11 asshown in FIG. 6. Here, the ceramics honeycomb structure 13 isincorporated in the catalyst converter container 11 with its outerperipheral surface being held by a ring 12. The material of ring 12 isnot particularly limited, and ordinarily a ring made of metallic mesh isused. It is preferred to provide a holding material 14 such as mat orcloth between the catalyst converter container 11 and the outerperipheral surface of the ceramics honeycomb structure 13.

Next, an embodiment of the present invention (the third invention) willbe explained. One embodiment of the third invention is a method formanufacturing the ceramics honeycomb structure having thecharacteristics as aforementioned, characterized in that a driedsubstrate having a honeycomb structure before firing is obtained using aclay mainly composed of a ceramics material, the substrate is coated andimpregnated with a reinforcing agent mainly composed of a compoundhaving in its structure at least one element selected from the groupconsisting of Si, Ti, Mg and Al, and thereafter the substrate is fired.The details will be explained below.

According to this embodiment, since firing of the substrate and gradualincrease (gradual decrease) of the porosity per unit volume (cm³) aresimultaneously performed by one firing, improvement of productivity andreduction of cost can be attained. Further, there is used a reinforcingagent mainly composed of a compound having in its structure an elementcontributing to gradual increase (gradual decrease) of the porosity ofthe resulting ceramics honeycomb structure, namely, element contributingto improvement of strength, specifically, at least one element selectedfrom the group consisting of Si, Ti, Mg and Al. Here, assuming a casewhere a water-soluble organic binder is used in making the substrate,even when the substrate (a dried product before firing) is coated andimpregnated with the reinforcing agent, the water-soluble organic binderdoes not dissolve and swell since the compound is hydrophobic, and thusdeformation of partition walls such as distortion of cells are notcaused. Therefore, a method including a step of making the substrate byadding the water-soluble organic binder to the clay is especiallypreferred, and by this method, there can be produced a ceramicshoneycomb structure having no problems such as deformation.

Moreover, in this embodiment, since a reinforcing agent mainly composedof a compound having in its structure an element contributing toimprovement of strength, the element is uniformly distributed throughthe whole substrate due to its physicochemical properties. Therefore, aceramics honeycomb structure uniformly improved in strength can beproduced without particular measures such as dispersion, and localoccurrence of erosion can be nearly avoided. Furthermore, by using thereinforcing agent as mentioned above, ceramics honeycomb structureshaving no variation in erosion resistance among products, and excellentin erosion resistance can be stably obtained through simple steps.

Next, an embodiment of the present invention (the fourth invention) willbe explained. One embodiment of the fourth invention is a method forproducing a ceramics honeycomb structure, characterized in that a driedsubstrate having a honeycomb structure before firing is obtained using aclay mainly composed of a ceramics material, the substrate is fired toobtain a fired substrate, the resulting fired substrate is coated withand impregnated with a reinforcing agent mainly composed of a compoundhaving in its structure at least one element selected from the groupconsisting of Si, Ti, Mg and Al, and thereafter the substrate is firedagain.

That is, not the substrate (dried substrate before firing) is coated andimpregnated with the reinforcing agent, but a previously fired-substrateis coated and impregnated with the reinforcing agent, and then thesubstrate is re-fired, and according to this method, a ceramicshoneycomb structure having substantially no problems such as deformationand improved in strength can be produced. In addition, ceramicshoneycomb structures having no variation in erosion resistance amongproducts, and excellent in erosion resistance can be stably obtainedthrough simple steps. Each step will be explained below.

In the embodiments of the third and fourth inventions, a substrate whichis a dried product having a honeycomb structure, specifically, asubstrate which is a dried product having a honeycomb structure and aplurality of cells forming a fluid flow passage partitioned by partitionwalls in the form of honeycomb is molded using a clay mainly composed ofa ceramics material.

The ceramics materials are not particularly limited, and there may beused, for example, at least one material selected from the groupconsisting of silicon carbide, boron carbide, titanium carbide,zirconium carbide, silicon nitride, boron nitride, aluminum nitride,alumina, zirconia, a raw material convertible into cordierite, aluminumtitanate and sialon. The relation between the kind of the ceramicsmaterial and that of the reinforcing material will be mentionedhereinafter.

In the embodiments of the third and fourth inventions, if necessary, theclay may contain other additives. Specifically, there may be containedwater-soluble organic binders, crystal growth assistants, dispersants,pore forming agents, etc. The water-soluble organic binders include, forexample, hydroxypropylmethyl cellulose, methyl cellulose, hydroxyethylcellulose, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl acetal,etc. The crystal growth assistants include, for example, magnesia,silica, yttria, iron oxide, etc., and the dispersants include, forexample, ethylene glycol, dextrin, fatty acid soaps, polyalcohols, etc.Moreover, the pore forming agents include, for example, graphite, wheatflour, starch, phenolic resins, polyethylene terephthalate, etc. Theseadditives can be used each alone or in admixture of two or moredepending on the purpose.

The clay can be prepared by usual methods, and, for example, it can beobtained by adding additives such as water-soluble organic binder to aceramics material to prepare a raw material, adding thereto water in asuitable amount, and, if necessary, adding other additives, followed bykneading the mixture by a kneader, a pressure kneader, a vacuum kneaderor the like.

In the embodiments of the third and fourth inventions, there are also nolimitations in the method for molding the substrate having a honeycombstructure, but extrusion molding is preferred from the point ofexcellent mass-productivity, and it is preferred to carry out theextrusion molding using extrusion molding apparatuses such as ram typeextrusion molding apparatus and twin-screw type continuous extrusionmolding apparatus.

Furthermore, the thickness of the partition walls forming cells is alsonot particularly limited, and, for example, even if the thickness ofpartition walls is thin, namely, 0.05 mm or less, there can be obtaineda ceramics honeycomb structure excellent in physical strengths.

In the embodiments of the third and fourth inventions, the substratewhich is a dried substrate before firing (unfired substrate) or thefired substrate is coated and impregnated with a given reinforcing agentshown below. In the embodiments of the third and fourth inventions, thesimilar reinforcing agent can be used at any stages. Therefore, thefollowing explanation is made of a case where a substrate which is adried substrate before firing is coated and impregnated with thereinforcing agent, but the same can be applied to the case where thefired substrate is coated and impregnated with the reinforcing agent.

As the reinforcing agents, there are used those which are mainlycomposed of a compound having in its structure such an element whichreduces the melting point of the ceramics material constituting thesubstrate or penetrates into pores in the substrate to reduce the volumeof the pores, thereby densifying the substrate and contributing to theimprovement of strength of the resulting ceramics honeycomb structure.As such elements, mention may be made of at least one element selectedfrom the group consisting of Si, Ti, Mg and Al. The compound which is amain component of the reinforcing agent is preferably one which producesan inorganic oxide upon burning.

Of the compounds as a main component of the reinforcing agent, thecompounds having Si in the structure are preferably organic compoundshaving a siloxane bond, and preferred examples thereof are silicone oil,silicone varnish, silicate alkoxy oligomer, mixtures thereof, etc. Asthe compound having Si in the structure, silica (SiO₂) can be used, butsince silica (SiO₂) per se is not liquid, it must be dispersed in adispersion medium before use.

Examples of the silicone oil include, for example, dimethyl siliconeoil, methylphenyl silicone oil, methyl hydrogen silicone oil,amino-modified silicone oil, epoxy-modified silicone oil,carboxyl-modified silicone oil, carbinol-modified silicone oil,methacryl-modified silicone oil, mercapto-modified silicone oil,phenol-modified silicone oil, one-terminal reactive silicone oil,different-functional groups-modified silicone oil, polyether-modifiedsilicone oil, methylstyryl-modified silicone-oil, alkyl-modifiedsilicone oil, higher fatty acid ester-modified silicone oil, etc.

Among the compounds as a main component of the reinforcing agents,preferred examples of those which have Ti and/or Al in the structure arealuminate alkoxy oligomers such as acetoalkoxyaluminum diisopropyrate,titanate alkoxy oligomers used as coupling agents, etc.

Among the compounds as a main component of the reinforcing agents, theremay be used, for example, magnesia (MgO) as a compound having Mg in thestructure, but since magnesia (MgO) per se is not liquid, it must bedispersed in a dispersion medium before use.

In the embodiments of the third and fourth inventions, the reinforcingagents can be prepared by using the above compound each alone or inadmixture of two or more, and preferably two or more of the compoundsare used in admixture. When the reinforcing agent is prepared by usingtwo or more in admixture, since compounds having various viscosities canbe selected and mixed, viscosity of the reinforcing agent can beoptionally adjusted and thus it becomes easy to uniformly coat andimpregnate the substrate with the resulting reinforcing agent.Furthermore, by optionally selecting and mixing the compounds, thedegree of reinforcement can be controlled with securing the desiredthermal shock resistance, and desired erosion resistance can be imparteddepending on the thickness of partition walls, etc. Specifically, it ispreferred to use reinforcing agents prepared by mixing dimethyl siliconeoil with silicate alkoxy oligomer or methylhydrogen silicon oil.

In the case of these reinforcing agents, the mixing ratio (SAO orMHSO/DMSO) of silicate alkoxy oligomer (SAO) or methyl hydrogen siliconeoil (MHSO) and dimethyl silicone oil (DMSO) is preferably 10/90-75/25(mass ratio), more preferably 15/85-50/50 (mass ratio), furtherpreferably 20/80-50/50 (mass ratio), especially preferably 25/75-50/50(mass ratio). When the mixing ratio is within the above range, ahoneycomb structure excellent in erosion resistance can be obtained withsecuring the desired thermal shock resistance.

Moreover, the reinforcing agents used in the embodiments of the thirdand fourth inventions may be those which are prepared by diluting thecompound such as silicone oil or the like with a diluent containing oneor two or more of aromatic hydrocarbons such as toluene and xylene,aliphatic hydrocarbons such as petroleum ethers and kerosene, petroleumhydrocarbons such as kerosene and light oil, alcohols such as isopropylalcohol, lauryl alcohol and butanol, and volatile silicone oils. Byusing such diluent, the degree of reinforcement can be optionallycontrolled and further the viscosity of reinforcing agent can beoptionally adjusted, and thus the substrate can be uniformly coated andimpregnated with the thus obtained reinforcing agent.

The reinforcing agents used in the embodiments of the third and fourthinventions have an absolute viscosity of preferably 1-10000 mPa·s, morepreferably 10-1000 mPa·s. In general, the compounds of low viscosity aresmall in polymerization degree and easily volatilized, and if theviscosity is lower than 1 mPa·s, when the partition walls are coated andimpregnated with the reinforcing agent and fired, generation of CO₂ andevaporation of H₂O occur, and simultaneously the effective componentsuch as Si present in the reinforcing agent is also volatilized, and asa result it becomes difficult to impart strength. On the other hand, ifthe viscosity exceeds 10000 mPa·s, it becomes difficult to coat andimpregnate the substrate with the reinforcing agent at a uniformthickness.

Furthermore, as for the compounds which constitute a main component ofthe reinforcing agents used in the embodiments of the third and fourthinventions, it is preferred to select suitable one depending on the kindof the ceramics raw materials For example, in the case of a clay mainlycomposed of a raw material convertible into cordierite, it is preferredto use a reinforcing agent mainly composed of a compound having Si inthe structure, specifically a reinforcing agent mainly composed ofsilicone oil or the like.

For coating and impregnating the substrate with the reinforcing agent,the substrate can be impregnated with the reinforcing agent prepared inthe form of liquid or slurry. Specifically, in order to produce aceramics honeycomb structure in which the porosity per unit volume (cm³)gradually increases from the inflow end part side to the outflow endpart side, a sufficient amount of the reinforcing agent is prepared andthe whole of the substrate is dipped therein, whereby the substrate iscoated and impregnated with the reinforcing agent, and thereafter acompressed air is blown thereinto from the outflow end part side togradually increase the amount of the reinforcing agent applied to thesubstrate towards the inflow end part side. The reinforcing agent may becoated on the substrate by spray coating, followed by blowing acompressed air thereinto from the outflow end part side to graduallyincrease the amount of the reinforcing agent applied thereto towards theinflow end part side.

In order to produce a ceramics honeycomb structure in which the porosityper unit volume (cm³) gradually decreases from the central part of asection perpendicular to the flow passage direction of cells towards theouter peripheral part, the substrate can be coated and impregnated withthe reinforcing agent, and can be impregnated with the reinforcing agentprepared in the form of liquid or slurry. That is, in order to produce aceramics honeycomb structure in which the porosity per unit volume (cm³)gradually decreases from the central part of a section perpendicular tothe flow passage direction of cells towards the outer peripheral part, asufficient amount of the reinforcing agent is prepared and the whole ofthe substrate is dipped therein, whereby the substrate is coated andimpregnated with the reinforcing agent, and then a compressed air or thelike is blown thereinto from the inflow end part side or the outflow endpart side to remove the reinforcing agent applied in an excess amount bythe compressed air and the like. Specifically, the ceramics honeycombstructure can be produced by gradually reducing the pressure of theblown compressed air from the central part of a section perpendicular tothe flow passage direction of cells towards the outer peripheral part,reducing the amount of the blown compressed air or shortening theblowing time.

Furthermore, the ceramics honeycomb structure in which the porosity perunit volume (cm³) gradually decreases from the central part of a sectionperpendicular to the flow passage direction of cells towards the outerperipheral part can also be produced by a method of stepwise coating andimpregnation of the reinforcing agent which is gradually increased inconcentration from the central part of a section perpendicular to theflow passage direction of cells towards the outer peripheral part.

Then, the substrate coated and impregnated with the reinforcing agent isfired. The substrate and reinforcing agent are preferably previouslydried before firing. The drying methods include air-flow drying, hot-airdrying, microwave drying, etc. The firing conditions are preferablyoptionally and suitably selected depending on the kinds of substrate andreinforcing agent. For example, when the substrate is mainly composed ofa cordierite-based material and the reinforcing agent is mainly composedof a compound having Si in the structure, such as silicone oil, thefiring may be carried out at 1300-1500° C.

On the other hand, the firing conditions for coating and impregnatingwith the reinforcing agent the substrate in the state of fired substrateafter firing can be in accordance with conventional methods, and can beoptionally selected depending on the kind of the substrate. For example,when the substrate is mainly composed of a cordierite-based material,the firing may be carried out at 1300-1500° C. Moreover, the refiringconditions for carrying out refiring after the fired substrate is coatedand impregnated with the reinforcing agent can be the firing conditionsin the case of coating and impregnating the substrate with theimpregnating agent as mentioned above. Specifically, the reinforcingagent is preferably previously dried before refiring, and preferablyoptional and desired firing conditions are selected depending on thekinds of substrate and reinforcing agent.

By carrying out the above steps, a ceramics honeycomb structure havingan excellent erosion resistance of the partition walls positioned in theinflow end part and a high compressive strength (isostatic strength) atthe time of canning can be simply produced without resulting invariation in characteristics among products. Furthermore, in thisceramics honeycomb structure, the porosity per unit volume (cm³)gradually increases from the inflow end part side to the outflow endpart side or gradually decreases from the central part of a sectionperpendicular to the flow passage direction of cells towards the outerperipheral part.

The present invention will be explained more specifically by thefollowing examples, which should not be construed as limiting theinvention in any manner.

Columnar ceramics honeycomb structures having a partition wall thicknessof 50 μm, a diameter of 100 mm and a height of 100 mm, having tetragonalcells at a density of 140 cells/cm², and having an opening ratio of 88%were produced by the following procedures (Examples 1-13 and ComparativeExamples 1-4). All the produced ceramics honeycomb structures had nocatalyst (supporting no catalyst).

EXAMPLES 1-10

A mixture obtained by mixing 100 parts by mass of a ceramics rawmaterial comprising a raw material convertible into cordierite with 8parts by mass of methyl cellulose, 0.5 part by mass of potassiumlaurate, 2 parts by mass of polyether and 28 parts by mass of water wasintroduced into a continuous extrusion molding machine to prepare asubstrate having a honeycomb structure (unfired substrate).

The resulting substrate was wholly dipped in a reinforcing agent shownin Table 1 to coat and impregnate the whole substrate with thereinforcing agent. Then, a compressed air was blown thereinto from theoutflow end part side to gradually increase the amount of the coatedreinforcing agent towards the inflow end part side, and simultaneouslyto remove excessively coated reinforcing agent, followed by firing at1400° C. for 4 hours to produce a ceramics honeycomb structure (Examples1-10).

EXAMPLE 11

A ceramics honeycomb structure was produced in the same manner as inExamples 1-10, except for using a reinforcing agent prepared by adding0.5 part by mass of a dispersant (polyoxyalkylene-based polymermanufactured by Nippon Oil & Fats Co., Ltd.: MARIALIM AKM-0531) to 100parts by mass of a dispersion obtained by dispersing 5% by mass ofsilica (SiO₂) powder in kerosene as a dispersion medium (Example 11).

EXAMPLE 12

A ceramics honeycomb structure was produced in the same manner as inExamples 1-10, except that the substrate having a honeycomb structure(unfired substrate) was fired at 1400° C. for 4 hours before whollydipping in the reinforcing agent and the resulting fired substrate waswholly dipped in the reinforcing agent shown in Table 1 (Example 12).

EXAMPLE 13

A ceramics honeycomb structure was produced in the same manner as inExamples 1-10, except that the amount of the reinforcing agent appliedwas gradually increased from the central part of a section perpendicularto the flow passage direction of cells towards the outer peripheral partby making a difference in air blow pressure between the inner peripheralpart and the outer peripheral part in blowing the compressed air fromthe flow passage end part side, and the excessively applied reinforcingagent was removed (Example 13).

COMPARATIVE EXAMPLE 1

A columnar ceramics honeycomb structure was produced in the same manneras in Examples 1-10, except that the substrate was not wholly dipped inthe reinforcing agent (no treatment with the reinforcing agent)(Comparative Example 1).

COMPARATIVE EXAMPLE 2

A columnar ceramics honeycomb structure was produced in the same manneras in Examples 11, except that the substrate having a honeycombstructure (unfired substrate) was dipped in the reinforcing agent up toa depth of 5 mm from the opening end face to coat and impregnate thepartition walls positioned in the vicinity of the inflow end part withthe reinforcing agent in place of wholly dipping the substrate having ahoneycomb structure (unfired substrate) in the reinforcing agent(Comparative Example 2).

COMPARATIVE EXAMPLE 3

A ceramics honeycomb structure was produced in the same manner as inComparative Examples 1 (Comparative Example 3).

COMPARATIVE EXAMPLE 4

A ceramics honeycomb structure was produced in the same manner as inExample 13, except that an extremely large difference in air blowpressure between the inner peripheral part and the outer peripheral partwas made in blowing the compressed air from the flow passage end partside, specifically, the air blow pressure was greatly increased withprogress from the inner peripheral part to the outer peripheral part(Comparative Example 4).

The physical properties of the resulting ceramics honeycomb structures(Examples 1-13 and Comparative Examples 1-4) were measured by thefollowing methods and the characteristics were evaluated. The resultsare shown in Tables 1 and 2. The porosity (%) in the sampling part (mm)of the ceramics honeycomb structures of Example 3 and ComparativeExamples 1 and 2 is shown in Table 3, and a graph in which porosity (%)is plotted against the sampling parts (mm) is shown in FIG. 3.

1. Porosity

(1) FIG. 1 is a schematic side view of a ceramics honeycomb structurewhich shows the sampling parts (A-E), and FIG. 2 is a schematic view ofa section perpendicular to the flow passage direction of cells whichshows the sampling parts (F-J). As shown in FIG. 1 and FIG. 2, thesampling parts A-J of the ceramics honeycomb structure 1 which aresamples to be measured were cut out at a given length (A-E: thickness 5mm, F-J: width 10 mm) (A: 2.5 mm (at a position of 0-5 mm from theinflow end part side), B: 25 mm (at a position of 22.5-27.5 mm from theinflow end part side), C: 50 mm (at a position of 47.5-52.5 mm from theinflow end part side), D: 75 mm (at a position of 72.5-77.5 mm from theinflow end part side), E: 97.5 mm (at a position of 95-100 mm from theinflow end part side), F: 5 mm (at a position of 0-10 mm from thecentral part of a section perpendicular to the flow passage direction ofcells), G: 15 mm (at a position of 10-20 mm from the central part of asection perpendicular to the flow passage direction of cells), H: 25 mm(at a position of 20-30 mm from the central part of a sectionperpendicular to the flow passage direction of cells), I: 35 mm (at aposition of 30-40 mm from the central part of a section perpendicular tothe flow passage direction of cells), J: 45 mm (at a position of 40-50mm from the central part of a section perpendicular to the flow passagedirection of cells)). In FIG. 1, the tip part of the inflow end partside is a position of 0, and the tip part side of the outflow end partis a position of 100 mm. In FIG. 2, the central part viewing from theoutflow end part side is a position of 0, and the outer peripheralsurface is a position of 50 mm.

(2) The sample for measurement which was cut out was dried at 150° C.for 2 hours and put in a container, which was set in the apparatus.

(3) Mercury was poured into the container, a pressure corresponding to aspecified pore diameter was applied, and volume of mercury absorbed inthe sample was obtained.

(4) A pore distribution was obtained by calculating from the pressureand the volume of mercury absorbed.

(5) A pore volume was obtained by calculating from the volume of mercuryabsorbed upon application of a pressure of 68.6 MPa (700 kgf/cm²).

(6) The porosity was obtained by the following formula from the totalpore volume.

Porosity (%)=total pore volume (per 1 g)×100/(total pore volume (per 1g)+1/2.52).

2. Erosion Resistance

A metal can in which a ceramic honeycomb structure was held and storedwas connected to an exhaust port of a gasoline engine of 4 cylinders(in-line) and 1.8 liter in cylinder volume. That is, the sample wasdisposed close to the engine. Then, the engine was operated under theconditions shown in FIG. 7, and 0.1 g of abrasive grains (siliconcarbide, GC320, average particle diameter 50 μm) were introduced whenthe number of revolutions reached 6000 rpm. Furthermore, the operationof engine was continued under the conditions shown in FIG. 7, andabrasive grains were introduced once per two cycles (one cycle being 130seconds) and this was continuously repeated. Several tests wereconducted with changing the total introduction amount of the abrasivegrains from about 2 g to about 16 g, and from the results, the erosionamount (air erosion volume) of the ceramics honeycomb structure when theintroduction amount of abrasion grains was 10 g was calculated.

The erosion amount was determined in the following manner. A rubbersheet was wound round the working end face on the side of measurement ofthe erosion amount of the ceramics honeycomb structure 1 and ceramicsbeads 20 having a diameter of 1.5 mm were laid compactly in thestructure at a height of about 3 mm. Thereafter, the beads wererecovered and the beads volume was measured, and the difference betweenthe beads volume after the erosion test and that before the test wasobtained. This was conducted thrice and the average value was taken asthe erosion amount. Evaluation was conducted on three ceramics honeycombstructures obtained in each example and comparative example, and whenall of the erosion amounts exceeded 3 cc, this was evaluated to beunacceptable for practical use and is shown by X, when all of theerosion amounts were 2-3 cc, this is shown by ◯, and all of the erosionamounts were less than 2 cc, this is shown by ⊚.

3. Isostatic Strength

A pressure applied at break was measured by conducting the test inaccordance with Automobile Standard JASO M505-87, and this pressure wastaken as isostatic strength (kg/cm²).

4. Thermal Shock Resistance

The ceramics honeycomb structure was heated to a given temperature by anelectric furnace, and then taken out and put in an atmosphere of roomtemperature of 20° C. In both of the state of high temperature justafter taking out and the state after cooling (20° C.) with cold air,occurrence of defects such as cracks due to thermal shock was visuallyexamined. When no defects were seen, the heating temperature was furtherraised and the test was repeated up to a temperature at which defectsoccurred. The critical temperature at which occurrence of defects wasseen was ascertained, and the thermal shock resistance was evaluated.TABLE 1 Method Honey- for comb coating Thermal structure & ErosionIsostatic Shock to be Reinforcing agent impreg- resistance strengthResistance treated Mix- nating (n 3) (n 5) (n 5) with ing with Amount(kg/cm²) Critical rein- ratio rein- Porosity (%) of Aver- tem- forcing(mass forcing Sampling portion erosion Value age perature agent Kindratio) agent A B C D E (cc) Evaluation measured value (° C.) Example 1Unfired *1 DMSO *2 100 Whole 24 24.5 25.5 26.5 27 2.2˜2.5 ◯ 21˜26 23 800dipping Example 2 Unfired *1 MHSO *4/ 10/90 Whole 24 25 25.5 26 26.52.0˜2.4 ◯ 22˜24 23 800 DMSO *3 dipping Example 3 Unfired *1 MHSO *4/20/80 Whole 20 21.5 23 24 25 1.2˜1.5 ⊚ 22˜26 25 800 DMSO *3 dippingExample 4 Unfired *1 MHSO *4/ 50/50 Whole 12.5 14 15.5 17 19 0.3˜0.6 ⊚29˜31 30 780 DMSO *3 dipping Example 5 Unfired *1 MHSO *4/ 10/90 Whole24 25 26 26.5 27 2.1˜2.3 ◯ 21˜24 22 800 Kerosene *6 dipping Example 6Unfired *1 MHSO *4/ 20/80 Whole 20.5 21.5 23 24 24.5 1.3˜1.8 ⊚ 23˜29 26800 Kerosene *6 dipping Example 7 Unfired *1 MHSO *4/ 50/50 Whole 1314.5 15.5 17 18.5 0.4˜0.7 ⊚ 30˜33 32 790 Kerosene *6 dipping Example 8Unfired *1 SAO *5/DMSO *3 10/90 Whole 25 25.5 26 26.5 26.5 2.5˜2.9 ◯21˜24 22 800 dipping Example 9 Unfired *1 SAO *5/DMSO *3 20/80 Whole22.5 23.5 24.5 25.5 27 2.0˜2.5 ⊚ 22˜27 24 800 dipping Example Unfired *1SAO *5/DMSO *3 50/50 Whole 17 18.5 19.5 21 22.5 0.7˜1.2 ⊚ 23˜27 26 80010 dipping Example Unfired *1 SiO₂ powder/  5/95 Whole 19 21 23 24.5 251.1˜1.7 ⊚ 22˜26 23 780 11 Kerosene *6 dipping Example Fired MHSO *4/20/80 Whole 20 21.5 22.5 23.5 25 1.2˜1.6 ⊚ 23˜26 25 800 12 DMSO *3dipping Comp. — None — — 27.5 28 27.5 27.5 28 4.2˜5.1 X 18˜23 20 800Exam. 1 Comp. Unfired *1 SiO₂ powder/  5/95 Dipping 20 27 28 27.5 281.6˜2.2 ◯ 17˜23 20 690 Exam. 2 Kerosene *6 of 5 cm from end face*1: Dried structure before firing*2: Dimethyl silicone oil (manufactured by Shin-Etsu Chemical Co. Ltd.,Trade name: KF96-100 CS, absolute viscosity: about 100 mPa · s)*3: Dimethyl silicone oil (manufactured by Shin-Etsu Chemical Co. Ltd.,Trade name: KF96L-0.65 CS, absolute viscosity: 0.65 mPa · s)*4: Methyl hydrogen silicone oil (manufactured by Shin-Etsu Chemical Co.Ltd., Trade name: KF99, absolute viscosity: 20 mPa · s)*5: Silicone alkoxy oligomer (manufactured by Shin-Etsu Chemical Co.Ltd., Trade name: KR-500, absolute viscosity: 0.65 mPa · s)*6: Petroleum hydrocarbon (manufactured by Nisseki-Mitsubishi Co. Ltd.,Trade name: Kerosene/Kurisef oil F8 mixed oil (main component: kerosene)

TABLE 2 Honeycomb Thermal structure Erosion Isostatic Shock to beReinforcing agent resistance strength Resistance treated (n 3) (n 5) (n5) with Mixing Porosity (%) Amount of (kg/cm²) Critical reinforcingratio Sampling portion erosion Value Average temperature agent Kind(mass ratio) F G H I J (cc) Evaluation measured value (° C.) ExampleUnfired *1 MHSO *2/ 20/80 25.0 23.5 22.0 21.0 20.0 1.6˜2.0 ◯ 24˜27 26800 13 DMSO *3 Comp. — None — 27.5 27.5 27.5 28.0 28.0 4.2˜5.1 X 18˜2320 800 Exam. 3 Comp. Unfired *1 MHSO *2/ 50/50 25.0 23.0 21.0 18.0 15.00.5˜1.2 ⊚ 23˜30 27 680 Exam. 4 DMSO *3*1: Dried structure before firing*2: Methyl hydrogen silicone oil (manufactured by Shin-Etsu Chemical Co.Ltd., Trade name: KF99, absolute viscosity: 20 mPa · s)*3: Dimethyl silicone oil (manufactured by Shin-Etsu Chemical Co. Ltd.,Trade name: KF96L-0.65 CS, absolute viscosity: 0.65 mPa · s)

TABLE 3 Porosity (%) Sampling portion (mm) 2.5 25 50 75 97.5 Example 320 21.5 23 24 25 Comparative 27.5 28 27.5 27.5 28 Example 1 Comparative20 27 28 27.5 28 Example 2(Evaluation)

As is clear from the results shown in Table 1, the ceramics honeycombstructures of Examples 1-12 were improved in erosion resistance andisostatic strength as compared with the ceramics honeycomb structure ofComparative Example 1, and were improved in isostatic strength andthermal shock resistance as compared with the ceramics honeycombstructure of Comparative Example 2. The reasons for the ceramicshoneycomb structures of Examples 1-12 being superior in erosionresistance to the ceramics honeycomb structure of Comparative Example 1can be considered that the porosity in the inflow end part (A) was lowerthan that in the outflow end part (E), and the porosity in the inflowend part (A) was 25% or lower in Examples 1-12 while it was high,namely, 0.27.5% and low in the part against which the exhaust gas struckin Comparative Example 1.

The reasons for the ceramics honeycomb structures of Examples 1-12 beingsuperior in isostatic strength to the ceramics honeycomb structures ofComparative Examples 1 and 2 can be considered that the whole of thehoneycomb structures showed a tendency of being lower in porosityalthough it is slight, specifically, the average of the porosity of thewhole ceramics honeycomb structures of Examples 1-12 was less than 26%while that of the ceramics honeycomb structures of Comparative Examples1 and 2 was 26% or higher and besides the proportion of the parts havinga porosity of 26% or higher was great. Furthermore, the ceramicshoneycomb structures of Examples 1-12 were superior in thermal shockresistance to the ceramics honeycomb structures of Comparative Examples1 and 2. It is considered that this is because the ceramics honeycombstructures of Examples 1-12 had no parts where the porosity greatlychanged and thus showed no abrupt change in the characteristics such asthermal expansion coefficient as is clear from the facts that, forexample, the gradual increasing rate of the porosity per unit volume(cm³) between the sampling parts (A)-(B) in the ceramics honeycombstructure of Comparative Example 2 was 0.28%/mm while the gradualincreasing rate of the porosity per unit volume (cm³) between thesampling parts (A)-(B) in the ceramics honeycomb structure of Example 11was 0.08%/mm, which means a gentle change of the porosity. Moreover, thereason can also be considered that as is clear from the results shown inTable 3 and FIG. 3, the generated heat stress and residual stress at thetime of production decreased due to the gentle gradual increase of theporosity from the inflow end part (A) to the outflow end part (E).

Furthermore, it can be seen from the results shown in Table 2 that theceramics honeycomb structure of Example 13 was improved in erosionresistance and isostatic strength as compared with the ceramicshoneycomb structure of Comparative Example 3. The reason for theceramics honeycomb structure of Example 13 being superior in erosionresistance to the ceramics honeycomb structure of Comparative Example 3can be considered that the ceramics honeycomb structure of Example 13was lower in porosity of the inflow end part as compared with theceramics honeycomb structure of Comparative Example 3 since the ceramicshoneycomb structure of Example 13 was produced by once wholly dipping inthe reinforcing agent like the ceramics honeycomb structures of Examples1-12. The reason for the ceramics honeycomb structure of Example 13being superior in isostatic strength to the ceramics honeycomb structureof Comparative Example 3 can be considered that the porosity of theouter peripheral part (J) was lower than that of the central part (F).Moreover, the ceramics honeycomb structure of Example 13 did not showdeterioration of thermal shock resistance as compared with the ceramicshoneycomb structure of Comparative Example 3. It is considered that thisis because the porosity gently gradually decreases from the central part(F) to the outer peripheral part (J) and hence the generated heat stressand the residual stress at the time of production did not increase.

Moreover, when Comparative Example 4 is compared with Example 13,Comparative Example 4 gave erosion resistance and isostatic strengthequal to or higher than that of Example 13, but the thermal shockresistance of Comparative Example 3 was extremely inferior. It isconsidered that this is because the gradual decreasing rate of theporosity per unit volume (cm³) from the central part (F) to the outerperipheral part (J) was greater as compared with Example 13.

INDUSTRIAL APPLICABILITY

As explained above, the ceramics honeycomb structure of the presentinvention has a structure in which a porosity per unit volume (cm³)gradually increases from the inflow end part side to the outflow endpart side at a rate lower than a given rate, and hence it has anexcellent erosion resistance, isostatic strength and thermal shockresistance.

Furthermore, there is provided a method for producing the ceramicshoneycomb structure according to which a substrate (a dried substratebefore firing) or a fired substrate having a honeycomb structureobtained using a given clay is coated and impregnated with a givenreinforcing agent and then fired (or refired) and this method hardlycauses defects such as distortion of partition walls and hardly givesvariation in physical properties between the products.

1-20. (canceled)
 21. A ceramics honeycomb structure formed of aplurality of cells forming a fluid flow passage partitioned by porouspartition walls, and comprising an inflow end part allowing fluid toflow therein, an outflow end part allowing fluid to flow therefrom, andan outer peripheral part including an outer peripheral surface,characterized by having a structure where a porosity per unit volume(cm³) gradually increases from the inflow end part side to the outflowend part side at a rate of 0.2%/mm or less.
 22. A ceramics honeycombstructure according to claim 21 which has a structure where the porosityper unit volume (cm³) gradually increases from the inflow end part sideto the outflow end part side at a rate of 0.1%/mm or less.
 23. Aceramics honeycomb structure formed of a plurality of cells forming afluid flow passage partitioned by porous partition walls, and comprisingan inflow end part allowing fluid to flow therein, an outflow end partallowing fluid to flow therefrom and an outer peripheral part includingan outer peripheral surface, characterized by having a structure where aporosity per unit volume (cm³) gradually decreases from the central partof a section perpendicular to the flow passage direction of the cells tothe outer peripheral part at a rate of 0.2%/mm or less.
 24. A ceramicshoneycomb structure according to claim 23 which has a structure wherethe porosity per unit volume (cm³) gradually decreases from the centralpart of a section perpendicular to the flow passage of the cells to theouter peripheral part at a rate of 0.1%/mm or less.
 25. A ceramicshoneycomb structure according to claim 21, wherein a porosity per unitvolume (cm³) in the area of up to 150 mm from the flow passage end faceof the inflow end part side in the inward direction of the flow passageis 10-50%.
 26. A ceramics honeycomb structure according to claim 22,wherein a porosity per unit volume (cm³) in the area of up to 150 mmfrom the flow passage end face of the inflow end part side in the inwarddirection of the flow passage is 10-50%.
 27. A ceramics honeycombstructure according to claim 23, wherein a porosity per unit volume(cm³) in the area of up to 150 mm from the flow passage end face of theinflow end part side in the inward direction of the flow passage is10-50%.
 28. A ceramics honeycomb structure according to claim 24,wherein a porosity per unit volume (cm³) in the area of up to 150 mmfrom the flow passage end face of the inflow end part side in the inwarddirection of the flow passage is 10-50%.
 29. A ceramics honeycombstructure according to claim 21, wherein the minimum thickness of thepartition walls is 0.030-0.076 mm.
 30. A ceramics honeycomb structureaccording to claim 23, wherein the minimum thickness of the partitionwalls is 0.030-0.076 mm.
 31. A ceramics honeycomb structure according toclaim 21 which comprises at least one ceramics selected from the groupconsisting of cordierite, alumina, mullite, silicon nitride, aluminumtitanate, zirconia and silicon carbide.
 32. A ceramics honeycombstructure according to claim 23 which comprises at least one ceramicsselected from the group consisting of cordierite, alumina, mullite,silicon nitride, aluminum titanate, zirconia and silicon carbide.
 33. Aceramics honeycomb structure according to claim 21, wherein the sectionperpendicular to the flow passage has a shape of circle, ellipse, oval,trapezoid, triangle, tetragon, hexagon or left and right asymmetricirregular shape.
 34. A ceramics honeycomb structure according to claim23, wherein the section perpendicular to the flow passage has a shape ofcircle, ellipse, oval, trapezoid, triangle, tetragon, hexagon or leftand right asymmetric irregular shape.
 35. A ceramics honeycomb structureaccording to claim 21, wherein the section of the cells perpendicular tothe flow passage has a shape of triangle, tetragon or hexagon.
 36. Aceramics honeycomb structure according to claim 23, wherein the sectionof the cells perpendicular to the flow passage has a shape of triangle,tetragon or hexagon.
 37. A ceramics honeycomb structure according toclaim 21 which is used as automobile exhaust gas purification catalystcarriers.
 38. A ceramics honeycomb structure according to claim 23 whichis used as automobile exhaust gas purification catalyst carriers.
 39. Aceramics honeycomb structure according to claim 21 which has a catalystcomponent supported on the partition walls and is incorporated into acatalyst converter by being held at the outer peripheral surface of theouter wall.
 40. A ceramics honeycomb structure according to claim 23which has a catalyst component supported on the partition walls and isincorporated into a catalyst converter by being held at the outerperipheral surface of the outer wall.
 41. A method for producing aceramics honeycomb structure formed of a plurality of cells forming afluid flow passage partitioned by porous partition walls, and comprisingan inflow end part allowing fluid to flow therein, an outflow end partallowing fluid to flow therefrom, and an outer peripheral part includingan outer peripheral surface, wherein a porosity per unit volume (cm³)gradually increases from the inflow end part side to the outflow endpart side at a rate of 0.2%/mm or less, which comprises drying asubstrate having a honeycomb structure extruded by using a clay mainlycomposed of a ceramics material, coating and impregnating a resultingsubstrate a reinforcing agent mainly composed of a compound having inits structure at least one element selected from the group consisting ofSi, Ti, Mg and Al, and thereafter firing thus treated honeycombstructure.
 42. A method for producing a ceramics honeycomb structureformed of a plurality of cells forming a fluid flow passage partitionedby porous partition walls, and comprising an inflow end part allowingfluid to flow therein, an outflow end part allowing fluid to flowtherefrom and an outer peripheral part including an outer peripheralsurface, wherein a porosity per unit volume (cm³) gradually decreasesfrom the central part of a section perpendicular to the flow passagedirection of the cells to the outer peripheral part at a rate of 0.2%/mmor less, which comprises drying a substrate having a honeycomb structureextruded by using a clay mainly composed of a ceramics material, firingthus dried substrate, coating and impregnating a resulting firedsubstrate with a reinforcing agent mainly composed of a compound havingin its structure at least one element selected from the group consistingof Si, Ti, Mg and Al, and thereafter firing thus treated substrateagain.
 43. A method for producing a ceramics honeycomb structureaccording to claim 41, wherein said compound is one which produces aninorganic oxide when it burns.
 44. A method for producing a ceramicshoneycomb structure according to claim 42, wherein said compound is onewhich produces an inorganic oxide when it burns.
 45. A method forproducing a ceramics honeycomb structure according to claim 43, whereinsaid compound has a siloxane bond.
 46. A method for producing a ceramicshoneycomb structure according to claim 44, wherein said compound has asiloxane bond.
 47. A method for producing a ceramics honeycomb structureaccording to claim 45, wherein said compound is a silicone oil, asilicone varnish, an alkoxy oligomer or a mixture thereof.
 48. A methodfor producing a ceramics honeycomb structure according to claim 46,wherein said compound is a silicone oil, a silicone varnish, an alkoxyoligomer or a mixture thereof.
 49. A method for producing a ceramicshoneycomb structure claim 41, wherein the reinforcing agent has anabsolute viscosity of 1-10000 mPa·s.
 50. A method for producing aceramics honeycomb structure claim 42, wherein the reinforcing agent hasan absolute viscosity of 1-10000 mpa·s.
 51. A method for producing aceramics honeycomb structure according to claim 41, wherein the ceramicsmaterial is a raw material convertible into cordierite.
 52. A method forproducing a ceramics honeycomb structure according to claim 42, whereinthe ceramics material is a raw material convertible into cordierite. 53.A method for producing a ceramics honeycomb structure according to claim41, wherein the clay contains a water-soluble organic binder.
 54. Amethod for producing a ceramics honeycomb structure according to claim42, wherein the clay contains a water-soluble organic binder.
 55. Amethod for producing a ceramics honeycomb structure according to claim53, wherein the water-soluble organic binder comprises at least onewater-soluble compound selected from the group consisting ofhydroxypropylmethyl cellulose, methyl cellulose, hydroxyethyl cellulose,carboxymethyl cellulose, polyvinyl alcohol and polyvinyl acetal.
 56. Amethod for producing a ceramics honeycomb structure according to claim54, wherein the water-soluble organic binder comprises at least onewater-soluble compound selected from the group consisting ofhydroxypropylmethyl cellulose, methyl cellulose, hydroxyethyl cellulose,carboxymethyl cellulose, polyvinyl alcohol and polyvinyl acetal.